As is known in the art, major ocean-going ships, civilian and military, have been using radars for navigation and collision avoidance for many decades. These radars display land masses, buoys, and other ships. At closer ranges and in heavier weather, sea surface return interferes with the ability to easily detect the objects the radar is designed to see. Mathematical models of sea clutter have been developed that aid in filter design, e.g., sensitivity time control (STC), to reduce the sea clutter without adversely affecting the primary performance of the radar. As is known in the art, STC is used to attenuate relatively strong signal returns from ground clutter targets in the first few range gates of the receiver. Without attenuation of such signals, the receiver would generally saturate due to the strong signal return.
The mathematical modeling of radar sea clutter has a long history. For example, one notoriously well known text discussing radar and sea clutter is Skolink, Merrill I., “Introduction to Radar Systems,” and particularly the discussion of Log-FTC receivers (McGraw-Hill, NY, 1984, pp 486-489). A more modern mean sea clutter model is provided in Barton and Ward, “Handbook of Radar Measurements,” Artech House, NY, 1985 (pp. 137-148). There are two complementary aspects to classic sea clutter modeling. The first aspect is the modeling of the clutter fluctuations from sample to sample. Usually, such fluctuations are modeled by a stationary stochastic process with a probability density function (pdf) that may differ significantly from author to author. A second aspect of sea clutter modeling is the nature of the mean clutter levels as a function of range.
There are disadvantages of conventional sea state modeling. For example, a reflectivity index is used in sea clutter modeling, however, this index has certain limitations. Reflectivity indexes are derived from averaging over many sea environments, many different wavelengths, and many wind aspects. In addition, below a critical grazing angle, further correction is required. Attempts to base STC on conventional sea clutter models have resulted in less than exemplary performance at short range while maintaining optimal performance at longer ranges. In the present, where there is more attention on relatively close targets/threats, where such targets can be quite small, improved STC filtering is very desirable.
In addition, conventional STC processing assumes that sea clutter is isotropic, i.e., the same in all directions. As is well known in the art, the purpose of STC processing is to de-clutter return at relatively close range. By reducing the effects of clutter, the ability of navigation and other radars to detect small cross section targets, such as small high speed boats, is enhanced.